docs(channels): add plugin developer guide and rename mock to plugin-example

- Add comprehensive developer guide for building channel plugins
- Add user-facing docs for installing/configuring custom channel plugins
- Replace custom-channels.md with new plugins.md
- Rename @qwen-code/channel-mock to @qwen-code/channel-plugin-example
- Add messageId field to Envelope type for response correlation

This provides clear documentation for developers building custom channel
adapters and renames the mock package to better reflect its purpose as
a reference implementation example.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

docs/developers/_meta.ts

@@ -17,6 +17,7 @@ export default {
     type: 'separator',
   },
 
+  'channel-plugins': 'Channel Plugin Guide',
   tools: 'Tools',
 
   examples: {